Method and a circuit for decoding four channel signals which are coded in a matrix and available in the form of a two-channel signal

ABSTRACT

By a method and a circuit for the purpose of decoding four-channel signals coded in a matrix and available in the form of a two-channel signal, whereby in each channel an automatic control of the amplification will take place by means of an output amplifier (ΔL, ΔR, ΔB, ΔC) in the output stage in question, it is proposed that the differential signal is conducted both to a delay circuit (ΔT) and then to an expander circuit (EXP), and also to a central rectifier element (D1), that the output signal (U CD ) form the rectifier circuit (D1, C A , C D ) is both conducted to the expander circuit (EXP) and applied for controlling the channels in pairs, whereby the stereo channels are controlled in phase and the center and background channels in reversed phase of the mean value (U CD ) of the differential signal, and that all channels moreover adjusted both in phase to the DC component of the summation signal amplitude and also in accordance with a level chosen in advance.

BACKGROUND OF THE INVENTION

The subject of the invention is a method and a circuit for the purpose,for instance on sound films, of decoding four channel signals, i.e. theright, left, centre and background, which are coded in a matrix and areavailable in the form of a two-channel signal, by the application ofamplifiers for the right and left channels respectively, and a summationapplifier forming the sum of the right and left stereo channels, for thecentre channel, and a differential amplifier forming the differencebetween the right and left stereo channels, for the background channel,whereby in each channel an automatic control of the amplification willtake place by means of an output amplifier in the output stage inquestion.

On sound films for stereo reproduction of the sound the two opticalsound tracks are placed between the frame field and the perforation,which ensures the synchronised transport of the films. On these soundtracks the right and left channels are recorded directly, and on top ofthese channels a mono signal such as the dialogue is recorded with equalamounts of information on both tracks. Effect sounds, such as pistolshots or screeching car tyres, are always recorded in stereo. FIG. 1shows a representation of a known system for the type referred to above,such as it is known. To obtain a greater stereo effect, a controllableamplifier has been inserted into each of the four channels. This oneincreases the amplification when the voltage over the capacitor Cincreases. In some cases D and C are replaced by a phase detector, whichmeasures the phase difference between the various channels and adjustsaccordingly. However, these systems all have the drawback that noiseimpulses, for instance in the right channel, which--on a sound film--ismost approximate to the perforation of the film, and which is thus mostexposed to scratches and dirt, will cause an upwards adjustment of theright channel, whereby the centre channel will be adjusted half as muchupwards, and an upwards adjustment of the background channel, as thisone is constantly affected by the difference between the right and leftchannels. This will cause a total displacement of the stero picture andis thus an illusion-spoiling drawback.

A circuit for the production of three channel signals is known from DEpatent specification No. 25.51.326, in which the centre channel isadjusted out of phase of the adjustment of the stereo channels right andleft, and in which delay devices in the stereo channels are to ensurethan the sound from the stereo channels does not reach the listenerbefore the sound from the centre channel reaches him. Such an out ofphase adjustment is inexpedient as, in case of small or no amplitudes onthe sound tracks, the amplification will be increased to maximum eitherin the centre channel, because the stereo channels are adjusted totallydown, or in the stereo channels, because the centre channel is adjustedtotally down. This will obviously give a deterioratedsignal-to-noise-ratio.

SUMMARY OF THE INVENTION

It is the task of this present invention to provide a method and acircuit of the type referred to initially, so that a simple circuit willconstantly ensure a stable stereo sound picture with an optimal signalto noise ratio under the given conditions.

This task is solved, according to the invention, in the way that thedifferential signal, which is produced as the difference between theright and left stereo channels, is conducted both to a delay circuit andthen to an input terminal of an expander circuit, and also, preferablythrough a filtration network, to a central rectifier element, that theoutput signal from the rectifier circuit is both conducted to anotherinput terminal of the expander circuit and is also applied forcontrolling the channels in pairs, whereby the stereo channels arecontrolled in phase and the centre and background channels in reversedphase to the mean value of the differential signal, and that allchannels moreover, by means of e.g. the DC component in the outputvoltage of the summation amplifier, which is produced as the sum of theright and left stereo channels, and/or another set point means, such asa potentiometer, are adjusted both in phase to the DC component of thesummation signal amplitude, and also in accordance with a level chosenin advance.

A circuit for performing the method, according to the invention, forderiving four channel signals from a recorded stereo signal, i.e. theright, left, centre and background, consisting of amplifiers for theright and left channels respectively, and a summation amplifier formingthe sum of the right and left stereo channels, for the centre channel,and a differential amplifier forming the difference between the rightand left stereo channels, for the background channel, whereby eachoutput amplifier automatically will control the amplification of theoutput stage in question, is characterised by that the output terminalfrom the differential amplifier is connected through a filtrationnetwork to a central rectifier element, whose input terminal is bothconnected to an inverting amplifier and also to the control inputterminal of an expander circuit for the background channel, and that theoutput terminal from the differential amplifier is connected to a delaycircuit, whose output terminal is connected to the expander for thebackground channel, and that the output terminal from the invertingamplifier is both connected, via a resistor, to the control inputterminals for the output stages of the centre and background channels,and also to the input terminals of a further inverting amplifier, whoseoutput is connected, via a resistor, to the control input terminals ofthe output stages for the stereo channels, and that the output terminalfrom the summation amplifier is connected via a low-pass filter to anon-inverting input terminal of a DC amplifier, whose inverting inputterminal is connected to a potentiometer serving as volume control, andwhose output terminal is connected, via a resistor, to the control inputterminals of the output stages for the centre and background channelsand, via a resistor, is connected to the control input terminals of theoutput stages for the stereo channels.

With the method and the circuit of the invention a number of advantagesare achieved, which either cannot, or can only with the application ofheavy resources, be provided with circuits already known. Thus obviousquality improvements may be obtained at the same time as both financialresources and mounting work are saved by:

(a) application of a central rectifier for the control of all fourchannels, where so far separate rectifiers have been used in eachchannel,

(b) application of a single volume control acting on all four outputstages, instead of four separate volume controls,

(c) placement of the delay circuit before the expansion, which willreduce the noise from the delay circuit,

and in that:

(d) the differential amplifier detects and amplifiers the differencebetween the right and left stereo channels, which difference is carriedon as the background signal and after rectification is used as controlvoltage both for the reversed phase control of the centre and backgroundchannels and for the in-phase control of the stereo channels and thesummation amplifier detects and aplifies the sum of the right and leftstereo channels, which sum is carried on as the centre signal and afterrectification it is used as control voltage for simultaneous in-phasecontrol of all channels,

(e) the rectifier is only supplied with frequencies in the frequenciesband covering speech communication, e.g. the frequency range from 100 Hzto 8 kHz, which has the result that minor phase differences and noiseimpulses will not affect the adjustment,

(f) the values of the resistors in the differential-signal controlledbranch of the government are far bigger than the values of the resistorsof hte sum-signal controlled control branch, preferably an order ofmagnitude of ten, whereby an undesired cross-control with mutual off-setis avoided,

(g) the rectifier has two capacitors, the attack capacitor, whichconnects the rectifier to the reference potential ensuring asufficiently rapid impulse response, and the decay capacitor, which isdesigned to keep the level of the output signal for a period that makescontrol possible, and which is also of such duration that there will beno frequent, unintended control interferences, and these two capacitorshave different capacitances, where the capacitance of the attackcapacitor is substantially smaller than the capacitance of the decaycapacitor, mainly an order to magnitude of ten, whereby any ripple onthe control voltage is reduced,

(h) the circuit will automatically effect the conversion from mono tostereo reproduction, whereas this conversion may also be obtained by theoperation of a single switch, which couples the input for the rectifiercircuit to the reference potential,

(k) the voltage for the rectifier is taken out before the delay circuit,thereby obtaining that the amplifier for the background channel isadjusted upwards before the signal passes through the delay circuit,i.e. that transients are also reproduced at the correct amplitude. Areasonable delay may be of e.g. the same duration as the time constantof the rectifier circuit, which is formed by the product between thedecay capacitor and an inherent resistance in the rectifier circuit.

By having the stereo channels controlled by the same voltage it isensured that the stereo picture does not flounder in case of noiseimpulses in e.g. one channel, and by using the DC component in theoutput voltage of the summation amplifier for simultaneous adjustment ofall four channels, the result will be an improved signal to noise ratio,and at the same time the drawbacks of the present reversed phase controlbetween the centre channel and the stereo channels are prevented, as theDC component will rise at a rising signal level in the recordingtechnique that is known under the disignation of "Noiseless Recording" ,which will be explained later. A further advantage in using the DCcomponent for controlling is that the sound track will open just beforethe modulation begins, which means that the adjustment has taken place,when the sound is to be reproduced. This characteristic would otherwiserequire a complicated rectifier with delay circuits in order to obtainthe same effect as is now provided by means of a capacitor and aresistor and a simple DC amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following by means of an executionexample shown on the drawing. The drawing shows, in:

FIG. 1 a schematic representation of a circuit such as it is basicallyknown,

FIG. 2 a schematic representation of a circuit according to theinvention, also with schematic representations of voltage courses in theessential junctions,

FIG. 3 the voltage course of the background channel in bigamplifications, where the inverting amplifier is excited so much that it"saturates",

FIGS. 4A and 4B an outline representation of a sound track to explainthe concept of "Noiseless Recording".

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a circuit according to the invention, in which the soundtracks, the photocells and the pre-amplifiers are schematicallyindicated. From these two signal sources, the amplifiers F1 and F2 formthe sum and the difference, respectively, between the two tracks. Thestereo channels are, before the amplifiers, carried out to therespective controllable stereo output stages. The output signal from theamplifier F1 is carried direct to the output stage ΔC for the centrechannel, where a branching from its input terminal is carried via alow-pass filter consisting of the capacitor C3 and and the resistor R7,to the DC amplifier F5, and via the resistors R5 will adjust the outputstages ΔL, ΔR for the stereo channels, and the output stages ΔC and ΔBfor the centre and background channels concurrently with the DCcomponent in a sound signal in the sound tracks. With this adjustmentthe signal to noise ratio will increase.

The output signal from the differential amplifier F2, which amplifiesthe difference between the two sound tracks, is conducted to the delaycircuit ΔT, which is inserted to ensure that the sound from the frontloudspeakers will reach the listener a little before the sound from thebackground loudspeakers reaches him. From the delay circuit ΔT thesignal is carried on to an expander circuit EXP, which is controlled bythe output voltage from the diode D1, which rectifies the band-passfiltered output signal from the amplifier F2. This band-pass filtrationwill, by means of the capacitor C1 and the resistor R1, cut off lowfrequencies, and by means of the capacitor C2 and the resistor R2 itcuts off high frequencies, such as noise. The signal is rectified willnow form the control voltage U_(CD) for the entire system. The attackcapacitor C_(A) will together with an inherent resistance R_(A) give asmall time constant (attack time), which provides a transientprotection, but also introduces a low-frequency ripple voltage. Thisripple voltage may be reduced by the application of the capacitor C_(D),which, together with an inherent resistance R_(D), give a long recoverytime, which stretches over considerably longer time than the timeconstant C_(A) ×R_(A). The control voltage U_(CD) will be conducted bothto the control input terminal for the expander circuit EXP, whose inputsignal is amplified proportionally to this control voltage as: ##EQU1##where R_(i), R_(A) and R_(O) are the input, attack and outputresistance, respectively, and where I_(i) is the input current andV_(ind) (AVG) is the rectified mean value of the input voltage, and alsoto the DC amplifier F3, which inverts the phase of the control signaland "saturates" app. 6 dB before the maximum input signal level. Theoutput signal from the DC amplifier F3 is conducted via the resistor R3to the control input terminal of the compressor circuit ΔB for thebackground signal. This output stage attenuates the output signal fromthe expander EXP, proportionally to the output voltage level from the DCamplifier raised to the power of 1/2, as: ##EQU2## where R_(i) R_(a) andR_(t) are the input, attack and feed-back resistances, respectively, andwhere I_(i) is the input current for the feed-back branch, and V_(ind)(AVG) is the rectified means value of the input voltage, so that thebackground channel is adjusted totally by an expansion and acompression. In the expander/compressor circuit EXP, ΔB the outputsignal may be provided with a bias in the form of a DC component, whosesize depends of an number of non-specified resistances. By this designof the amplification of the background channel, the amplification abovea certain level, e.g. -6 dB, will be proportional to the input signallevel of the background channel, which means that the signal levelexpressed in decibels will double.

For a further illustration of the signal levels of the backgroundchannel, see FIG. 3, which shows the amplification of the backgroundchannel such as it is totally controlled by the expander/compressorcircuits EXP, ΔB by means of the output signal U_(CD) from the diode D1.In case of a rising amplitude the input voltage to the amplifier F3 theoutput voltage will fall down to a level being app. 6 dB below themaximum excitation. Hereafter a saturation will occur in the amplifierF3, so that a further reduction of the control voltage is impossible.This will have the result that the compressor circuit can no longeraffect the signal, and therefore the output of the expander EXP, abovethis signal level, will only be attenuated at a constant factor. Theamplification of the expander/compressor circuits as a whole arepresented on the top curve of FIG. 3.

Moreover, FIG. 2 shows that the output signal from the amplifier F3 willequally be carried to the input terminal of a further inverting DCamplifier F4, which ensures that the stereo channels are controlledreversely to the centre channel. This control method is practical withthe circuit of the invention, as it is ensured in a way that will bedescribed below, that all outputs are reduced when there is no signal onthe sound tracks. Thereby one or more of the output terminals makingnoise is suitably avoided, when there is no signal source. The controlsignal for this control is provided in the way that the output signalfrom the summation amplifier F1 is low-pass filtered at a low upperlimiting frequency, so that it is essentially the DC component of thesum signal that is applied for the control. This DC component isconducted to the non-inverting input terminal of the DC amplifier F5,whose inverting input terminal is connected to a potentiometer P1 thatserves as a volume control, and whose output is connected via a resistorR6, both to the control input terminals of the output stages ΔC, ΔB forthe centre and background channels and also to the control inputterminals for the output stages, ΔL, ΔR for the stereo channels. Herebya volume control is suitably provided for all channels by means of onlyone potentiometer.

The reason that the DC component may be applied for adjustment purposesis the technique which was introduced already in 1920, under thedesignation of "Noiseless Recording", NR. FIG. 4A shows a schematicrepresentation of a sound track of a sound film, recorded without anyNR. The photocells will on average receive a constant light amountwherefore the DC component will always be close to nil. Because ofstatic electricity etc. there will always be dust grains deposited onfilms, which is of no special importance in the dark coloured areas. Theexposed area, where the light penetrates, will however be affected bythe dust particles deposited, as these will unsuitably affect the soundas noise. To avoid this drawback it was proposed to narrow the soundtrack in sound breaks during the recording to that only a small amountof light was transformed to the photocells, when there were no soundsignals in the sound tracks. A schematic representation of a sound trackwith NR is shown in FIG. 4B. It shows that the width of the sound trackis heavily reduced during sound breaks, and that the width is increasedto normal a short time before the sound break is ended. This change ofthe width has the result that the amount of light which the photocellsreceive on average is not constant, and therefore the output signal fromthe photocells during sound reproduction will comprise a DC component,and according to the invention, this will suitably by applied for theadjustment of the level of the power amplifiers.

The components C1, R1, C2, and R2 of the filtration network aredimensioned so that only frequencies in the speech range (100 Hz-8 kHz)are passed through to the rectifier circuit. The values of the resistorsR3 and R4 are far larger than the values of the resistors R5, R6,preferably on the order of magnitude of ten. Capacitance of thecapacitor CA is substantially smaller than the capacitance of thecapacitor CD in the rectifier circuit with the central rectifier elementD1, preferably on the order of magnitude of ten. An input terminal ofthe rectifier element D1 is preferably connected to a switch K having asecond connection which is linked to a reference potential. The delaycircuit ΔT is inserted between the differential amplifier F2 and theexpander circuit EXP. Voltage to the rectifier circuit D1, CA, CD istaken out before the delay circuit ΔT.

I claim:
 1. A method for decoding four channel signals, i.e. right,left, centre and background, which are coded in a matrix and areavailable in the form of a two-channel signal comprising left and rightchannels, by the application to amplifiers of the right (R) and left (L)channels respectively, and to a summation amplifier (F1) forming the sumof the right and left channels, for the centre channel, and to adifferential amplifier (F2) forming a difference between the right andleft channels, for the background channel,whereby in each channel anautomatic control of the amplification will take place by means of anoutput amplifier (ΔL, ΔR, ΔB, ΔC) in the output stage in question,comprising the steps of: conducting the difference signal to an inputterminal of an expander circuit (EXP) via a delay circuit, and to acentral rectifier element (D1); conducting an output signal (U_(CD))from the rectifier element (D1) to another input terminal of saidexpander circuit (EXP) and applying said output signal (U_(CD)) forcontrolling the channels in pairs; and adjusting all channels by meansof at least one of a DC component of an output voltage of the summationamplifier (F1) which is produced as the sum of the right and leftchannels and a set point means, both in phase according to the DCcomponent of the summation signal amplitude, and also in accordance witha level chosen in advance.
 2. A circuit for decoding four channelsignals, i.e. right, left, centre and background, which are coded in amatrix and are available in the form of a two-channel signal comprisingright and left channels, comprising amplifiers for the right (R) andleft (L) channels respectively, and a summation amplifier (F1), whichforms a sum of the right and left channels, for the centre channel, anda differential amplifier (F2) forming a difference between the right andleft channels, for the background channel,whereby in each channel anautomatic control of the amplification will take place by means of anoutput amplifier in the output stage in question, wherein an outputterminal from the differential amplifier (F2) is connected through afiltration network (C1, R1, C2, R2) to a central rectifier element (D1)having an output terminal both connected to an inverting amplifier (F3)and also to a control input terminal of an expander circuit (EXP) forthe background channel, the output terminal of the differentialamplifier (F2) is connected to a delay circuit (ΔT) having an outuputterminal which is connected to the expander circuit (EXP) for thebackground channel, an output terminal from the inverting amplifier (F3)is both connected, via a resistor (R3), to control input terminals ofoutput stages (ΔC, ΔB) for the centre and background channels and alsoto an input terminal of a further inverting amplifier (4) having anoutput terminal connected, via a resistor (R4), to control inputterminals of output stages (ΔL, ΔR) for the right and left channels, andan output terminal of the summation amplifier (F1) is connected, via alow-pass filter (R7, C3), to a non-inverting input terminal of a DCamplifier (F5) having an inverting input terminal connected to apotentiometer (P1) serving as volume control and an output terminalconnected, via a resistor (R6), to the control input terminals of theoutput stages (ΔC, ΔB) for the center and background channels and, via aresistor (R5), connected to the control input terminals of the outputstages (ΔL, ΔR) of the right and left channels.
 3. A circuit, accordingto claim 2, wherein the components (C1, R1, C2, R2) of the filtrationnetwork are dimensioned so that only frequencies in the speech range(100 Hz-8 kHz) are passed through to the rectifier circuit.
 4. Acircuit, according to claim 2, wherein the values of the resistors (R3,R4) are far larger than the values of the resistors (R5, R6).
 5. Acircuit, according to claim 2, wherein capacitance of a capacitor (CA)is substantially smaller than the capacitance of a capacitor (CD) in arectifier circuit with the central rectifier element (D1).
 6. A circuit,according to claim 2, wherein an input terminal of the rectifier element(D1) is connected to a switch (K) having a connection which is linked toa reference potential.
 7. A circuit, according to claim 2, wherein thedelay circuit (ΔT) is inserted between the differential amplifier (F2)and the expander circuit (EXP).
 8. A circuit according to claim 2,wherein voltage to the rectifier element (D1) is taken out before thedelay circuit (ΔT).
 9. The circuit of claim 5, wherein the capacitanceof capacitor (CA) is smaller by an order of magnitude of ten than thecapacitance of the capacitor (CD).
 10. The method of claim 1, comprisingthe additional step ofcoducting the difference signal through afiltration network (C1, R1, C2, R2) to the central rectifier element(D1).
 11. The method of claim 1, wherein said set point means is apotentiometer (P1).
 12. The method of claim 1, comprising the additionalstep ofcontrolling the right and left channels in phase and the centreand background channels in reversed phase to a mean value of the outputsignal (U_(CD)).
 13. The circuit of claim 4, wherein the values of theresistors (R3, R4) are larger by an order of magnitude of ten than thevalues of the resistors (R5, R6).